Opening a major conference marking the 20th anniversary of the Agreement, Senator Mitchell said: "These were ordinary men and women who after 700 days of failure they joined in one day of success and they changed the course of history."

Former US president Bill Clinton and ex-prime minister Tony Blair were also among those invited to attend the event taking place at Queen's University Belfast.

At one point Mr Mitchell, who had been a special envoy, asked those involved in reaching the Agreement to stand for lengthy applause from the audience packed into the Whitla Hall.

He said: "The real heroes of the Agreement were the people of Northern Ireland and their political leaders.

"The people supported the effort to achieve the Agreement and afterwards they voted overwhelmingly to ratify it.

"Their political leaders, in dangerous and difficult circumstances, after lifetimes devoted to conflict, summoned extraordinary courage and vision and reached agreement often at great risk to themselves, their families and their political careers.

"I ask you to join me in recognising the heroes and the peacemakers."

Northern Ireland has been without a functioning government since last January.

The devolved institutions at Stormont collapsed after the resignation of Martin McGuinness amid a row over a botched green energy scheme.

Since then divisions between the two biggest parties, the DUP and Sinn Fein, have widened to include Irish language rights, gay marriage and dealing with the legacy of the past.

A deal had been close but the talks process broke down on Valentine's Day and there have been further wranglings over leaked documents and their content.

Senator Mitchell added: "The current problems in Northern Ireland are difficult, they are serious and they must be resolved but at the same time we should not hold Northern Ireland to a higher standard to everywhere else in the world.

"What we must do now is not to despair, not to look backwards, but to reaffirm to the people and the leaders of Northern Ireland - our dedication to the principle of political differences here and elsewhere must be resolved through peaceful and democratic means, not through the use or threat of violence."